Field of the Invention
The disclosure relates generally to electrodes for obtaining biosignals from a recording surface, for example a skin of a subject.
Description of the Prior Art
When monitoring the vital signs or other physiological parameters of hospitalized patients, sensors are attached on patients' skin or catheters are inserted either into natural openings of the body or catheters are pierced through the skin. The common practice is to connect these sensors with electrically or optically conductive cables to measurement instruments. The instrument may reside either on bedside (e.g. multi-parameter patient monitors in operating room (OR) or in intensive care unit (ICU)) or it may be a relatively small box carried by the patient (e.g. ECG telemetry).
Recent technological development has made it possible to build battery-operated sensors, which include means for performing the actual measurement, converting the measured signals into digital format, and transmitting wirelessly the measurement data and/or calculated parameters to a host device. These devices are referred to as wireless sensors.
Wireless sensors provide obvious benefits for both caregivers and patients. The so-called ‘cable clutter’ has been recognized as one of the biggest issues in the care process of high-acuity patients. There are lots of cables in the hospitals that also create issues with infections and the usability. By using wireless sensors one can reduce the amount of cables used in a hospital and improve the usability of the different parameters and the total care process. It is not necessary to remove all the cables, but a significant improvement would be achieved by removing only a moderate number of leads or wires. This is because the tendency to tangle up increases disproportionally with the number of cables. The patient group, which would benefit most from the wireless sensors, is low-acuity patients. Being not physically tied to the patient monitor with lead wires, they are free to move around, like visiting the bathroom without assistance. Also in case of a small patient monitor carried by the patient, wireless sensors offer better reliability and are more comfortable for the patient.
Another aspect supporting the use of disposable sensors in hospital environment is the infection control, which has become a big issue. The term “disposable” as used herein refers to a single-use sensor which is used once and then disposed. Totally disposable sensors would make infection control easier. They would also streamline the care process by eliminating the need for cleaning the sensors. If one is able to use disposable single-patient-use sensors, it prevents the spreading of infections and cross contamination inside the hospital. This also improves the care process by saving time and money.
There are several disposable sensors available in the market such as a depth-of-anesthesia sensor, ECG electrodes, etc. There are also some wireless parameters available in the markets. Most of the wireless parameters are reusable or disposable with changeable or rechargeable battery. All of these variations require charging or special handling of the batteries. The also require special installation when starting to use the parameter because the battery needs to be added to the sensor separately. Difficult maintenance and high cost related to batteries has been the key factor in preventing wireless sensors from becoming widely accepted. Moreover, typically the batteries used needs to be recycled. In order to gain wide acceptance for the wireless sensors, the battery replacement cycle should match the hospital's daily routine and the disposed batteries and sensors should require no special handling
Disposable sensors that contain electrodes (such as ECG, EEG etc. . . . ) has limited shelf life for several reasons. Electrodes for measuring biosignals from a recording surface, for example a skin of a patient may be generally classified into dry electrodes or wet electrodes depending on the presence of an electrolyte on the surface attached to the skin. Dry electrodes are mainly applied to the skin using an elastic band. An example of a dry electrode is heart rate meter belt used in sports medicine. On the other hand, wet electrodes may be attached to the skin using a conductive liquid or solid gel to provide a continuous conductive path between the recording surface and the electrode sensing element. Conductive gels may contain a salt, such as KCl or NaCl, in order to achieve electrical current flow. The preferred gel is one with a high salt content, since such a gel produces a better conductor than that obtained when using a gel with low salt content. In addition, the use of a high salt content typically requires less skin abrasion at the time of application to reduce the impedance of the skin-electrode interface after subsequent electrode application. Consequently, biosignal measurement sensor electrodes with high salt content traditionally may have a limited shelf life (maximum storage time prior to use), for example, due to drying of the gel in the electrodes, and also due to the changes that may take place in the sensor materials. Wet gel electrodes provide better contact that dry electrodes: the contact impedance is lower and the signal bandwidth extends to lower frequencies. This is why dry electrodes are typically used in limited applications, such as heart rate measurement, whereas wet gel electrodes are used in diagnostic ECG, where various features of the signal are analysed.
For a long-term storage, e.g. up to about 12 months, wet sensors or sensor electrodes may be stored within an enclosure or a package, which provides a proper atmosphere to prevent the drying of the gel. For example, a pouch laminated with moisture and UV-proof materials, such as aluminum, may be used to prevent the drying of the gel. As a result there is an atmosphere inside the pouch where humidity may be about 99%. There may be salt (Chloride) present inside the pouch, since the electrolyte gels contains typically KCl or NaCl to enable good signal between the electrode and the tissue. Thus, the humidity and the existence of the salt inside the pouch creates an atmosphere which is really harmful for several materials. This atmosphere corrodes many materials and chlorides or oxidizes them and it requires special attention to the material selections of the sensor.